The present invention relates generally to a method and apparatus for converting a sampled signal of an analog input signal to a digital output signal.
Such a method for converting a sampled signal to a digital output value of L data bits includes the steps of converting the sampled signal to a digital reference value of M&lt;L data bits in length. The digital reference value is in turn converted back to an analog reference signal and subtracted from the sampled signal The resulting difference signal is amplified and converted to a digital partial value of M&lt;N&lt;L data bits in length. This digital partial value is summed with the reference value to yield the output value having L data bits. The higher-value data bits of the output value is formed by the reference value and the lower-value data bits of the output value is formed by the partial value. The sum of the number of data bits of the reference value and the partial value is not less than the number of data bits of the output value.
Analog/digital converters are disclosed in Electronic Design, Volume 35, pp. 119-122, "Comparing ADC architectures is a designer's best bet" and Funkschau, No. 7, Page 57-60, "Video in Bits". These converters function according to the above-described method (i.e. subranging technique). By dividing the conversion process into the formation of a reference value and a partial value, a total resolution is obtained, after the two values are brought together, which is greater than the partial resolution in each case. In this method, the reference value is a first approximation of the output value. The difference signal contains the data which is below the resolution limit of the reference value. Funkschau, No. 7 discusses problems and errors which can occur in this type of analog/digital conversion method. For example, in sound studios, these errors result in audible interference.
DE-A-2 026 643 discloses an analog/digital converter that uses a subranging method. A first digital partial result (i.e. a reference value) is first formed and stored from an electrical analog input value by an analog/digital converter of relatively low accuracy. This partial result is converted back by a digital/analog converter with the full, desired accuracy. From this partial result the difference to the input value is formed. Finally, after amplification of this difference value, a second digital partial result is obtained, which is brought together with the first partial result in a result register to form the output value. A similar analog/digital converter is also disclosed in DE-A 1 801 670.
The problem with the prior art is that interference occurs during the formation of the difference signal.